(1) Field of the Invention
The present invention relates to a method and to a rotary wing aircraft that are optimized so as to minimize the consequences for the occupants of said aircraft to an off-specification emergency landing including positive acceleration in a direction going from the rear end of the aircraft towards its front end and/or downwards, i.e. in a direction going from the rotary wing towards the ground.
(2) Description of Related Art
Such an aircraft has a rotary wing provided with at least one rotor that provides at least some of the lift of the aircraft, and that may also propel said aircraft if it is a helicopter.
A rotary wing then includes a plurality of blades. The person skilled in the art uses the term “advancing blade” for each blade that is advancing relative to the movement in translation of the aircraft, and uses the term “retreating blade” for each blade that is retreating relative to the movement of translation of the aircraft. Thus, depending on its azimuth position, over one-half revolution a blade is to be found in an advancing blade position, while over the next half-revolution said blade is to be found in a retreating blade position.
For convenience, and in the text below, it is assumed that the left of an aircraft is on the left of an individual who is sitting on the anteroposterior longitudinal plane of symmetry of the aircraft with the individual's back towards the rear of the aircraft so that the individual is looking towards the front of the aircraft. This amounts to considering that the right side of the aircraft is on the right-hand side of said individual.
Under such circumstances, when the rotor blades rotate in a clockwise direction when seen from above, the blades on the left of said individual are blades in an advancing position while the blades on the right of the individual are blades in a retreating position. Each blade passes in alternation from having the status of a blade in the advancing position to a status of a blade in the retreating position. Naturally, this effect is inverted if the rotor turns counterclockwise.
Conventionally, a rotary wing rotor is driven in rotation by a main gearbox, sometimes referred to by the acronym “MGB”, the main gearbox being driven by a power plant.
The main gearbox is then fastened to the structure of the aircraft by fastener elements.
The term “fastener element” is used to designate a member or a set of members that connect the main gearbox to the airframe.
For example, a fastener element may comprise a fastener bar and means for fastening said fastener bar firstly to the main gearbox and secondly to the airframe.
A fastener element may also be a portion of an element for transmitting torque. For example, one such element includes at least two fastener elements.
It can be understood that this list is not exhaustive, and that elements for fastening a main gearbox are nevertheless known to the person skilled in the art.
In one variant, the aircraft has four fastener elements, two front fastener elements and two rear fastener elements, for example, the front fastener elements being disposed symmetrically on either side of said anteroposterior longitudinal plane of symmetry, the rear fastener elements being disposed symmetrically on either side of said anteroposterior longitudinal plane of symmetry.
In another variant, the aircraft has three fastener elements, namely, by way of example: a front fastener element located close to or in the anteroposterior longitudinal plane of symmetry; and two rear fastener elements disposed symmetrically on either side of said anteroposterior longitudinal plane of symmetry; or indeed a rear fastener element disposed close to or in the anteroposterior longitudinal plane of symmetry, and two front fastener elements disposed symmetrically on either side of said anteroposterior longitudinal plane of symmetry.
Independently of the variant, in flight, the fastener elements transmit the lift of the rotor and the forces generated by said rotor to the structure of the aircraft. On the ground, the fastener elements then transmit the weight of the rotor plus the main gearbox to the structure of the aircraft.
With reference to document FR 2 474 996, the fastener elements may comprise a plurality of hinged oblique bars sometimes referred to as “suspension bars” or “MGB bars”.
Document FR 2 492 933 describes fastener elements for suspending a gearbox from a structure. Similarly, documents FR 1 507 306, US 2006/032973, FR 2 777 860, and EP 1 395 488 describe fastener elements for main gearboxes. Documents EP 1 291 545 and EP 0 488 845 are also known.
During an emergency landing, the impact that results from the aircraft making contact with a contact surface subjects the heavy members of the aircraft to accelerations, and in particular subjects the main gearbox to accelerations.
Regulations for certifying aircraft define the accelerations that should be taken into account when dimensioning the behavior of said heavy members during such emergency landings.
Under such circumstances, each fastener element is dimensioned so as to present strength in its own failure mode that enables it to withstand the emergency landing conditions as prescribed by said regulations, possibly with an additional safety margin or safety coefficient. This strength is referred to as the “prescribed strength” for convenience, for example a buckling prescribed strength for a fastener element that is stressed in compression.
For convenience, it should be observed that below the term “prescribed emergency landing conditions” is used to designate both emergency landing conditions as defined by certification regulations and also prescribed emergency landing conditions as such plus a safety margin added by the manufacturer, where applicable. Under such circumstances, the emergency landing conditions defined by the certification regulations, with or without any extra applied by the manufacturer, are referred to for convenience as “prescribed emergency landing conditions”.
Furthermore, emergency landing conditions that are not covered by said certification regulations are, in contrast, referred to as “off-specification emergency landing conditions”.
During an emergency landing with positive acceleration equally well in a direction going from the rear end towards the front end of the aircraft and/or downwards going from the rotary wing towards the ground, the mechanical assemblies including the power plant, the main gearbox, and the rotor tend to move towards the front end of the aircraft under the effect of the impact. This forward movement is nevertheless restrained by the fastener elements.
However, during an off-specification emergency landing, i.e. a landing presenting conditions that are more penalizing than the prescribed emergency landing conditions used by the manufacturer for dimensioning those fastener elements, it is possible that the fastener elements will break.
The main gearbox will then tend to tilt forwards or towards the side of the aircraft where the blades are in the advancing position, for example. As a result, the blades of the rotary wing are in danger of coming into contact with the ground. This can sometimes lead to the members that provide the blades with stiffness in flapping and in drag being broken. As a result of that, a blade coming into contact with the ground runs the risk of being caused to make very large movements that may cause it to come into collision with the cabin of the aircraft or with some other structural element of the aircraft, such as its tail boom.
More precisely, the blades in the advancing position run the risk of impacting against the cockpit, while the blades in the retreating position run the risk of coming into collision with the tail boom, for example.
Although a collision between a blade and the tail boom is likely to give rise a priori to damage that is purely structural, it will readily be understood that such contact between a blade and the cabin of the aircraft is unfortunately likely to injure the occupants of the cockpit.
Thus, in off-specification emergency landing conditions that are not specified by certification regulations, an accident may lead to a blade coming into collision with the cabin of the aircraft.
Pilots therefore naturally tend to cause the aircraft to roll towards the side having blades in the retreating position so as to avoid such a collision.